Tone control circuit



Oct. 16, 1951 SIGNAL SOURCE R. J. COWLES TONE CONTROL CIRCUIT Filed April 30, 1947 TO POWER SOURCE FIG?) LOAD CIRCUIT E OUTPUT FCPS ROBERT J. COWLES IN V EN TOR.

'ATTO RN EY Patented Oct. 16, 1951 UNITED STATES PATENT OFF-ICE Robert J-. Cowles, Fort Wayne, Ind., assignor, by

me's'ne assignments, to Farnsworth Research Corporation, a corporationof Indiana.

Application Atari 1,30, 1947 Serial No. 744,968

'1 Claims. 1

This invention relates to tone control circuits and more particularly to acircuit for controlling the high or low frequency response of an am plifier with but a single control.

Accordin to conventional practice, in radio receiver amplifiers and other audio frequency aniplifiers, there provided a'manuall yoperated tone 'cohtrol whereby thejhigh frequency or low frequency response of'theamplifir maybe modi fied by the operator. 'The tone control'usu'ally modifies thecharacter o f'th e amplifier output to render an acousticeffect which is pleasingto the oper itor, or may beadjusted for the reduction of hum hiss or. scratch in the output signal;

Various expedients, in the nature of amplifier tone controls, have heretofore been proposed in which a plurality of tone controls andmircu-its' are mplo ed. f a at y at e a ing. 'Q ac? centuating either the high, low or medium'r equencies and whereby, for instanee the' high freg quencie's may be attenuated; withcutaltering the 10w fre u n y. espon 9 re sabY Q'i the useoi a plurality of tone controls and assofciated circuits may be satisfactory. However; the d men her ay e asier c r uit, o the unskilled operator'andi construction involves relatively complicated and" expensive circuit ap'-' paratu Indirect coupled amplifier. circuits wherein a first amplifier tube anode is directly f llnefl d to; the grid of a' second amplifier tube, it islc'on'veri' tional to connect a tone .controlbetween the tube electrodes. It is known, however, that adjustf merit of 'such'a tone control results in'effe ng'a change in. the grid bias voltage applied t ojthe second amplifier tube. Understandably, the over-f all performance. of the direct coupled amplifier will then be adversely afiectedf a The object of theipres'ent invention, therefere, is to'provide a novel methodand-means for 'con trolling-the frequency respees 'oran audio em"- pli fi er with a single relatively simple combini- A further object of the present inventionis to provide ari'inexpensive tonecontrol system'which is simple and rlialole in operation are which will attenuate opera ion or sump fig u'ency] range without appreciably afiectingfoth t o r fl Another object of the present inventionisrto v tone; o r l. arrear f9?" ire t coupled amplifier circuits wherein grid biasfre mains constant regardless of the tpne control e 1 as gr ane hi Pr senter/sneer hat is providedan audio signal amplifier comprising first and second amplifier stages. For controlling the frequency range of amplified signals, there is provided a coupling circuit comprising a first impedance which is resonant in arelatively-high frequency range'and a second'impedance which is resonant in a relatively lowfrequency range. For adjusting-the frequency range ofsignals; an adjustable means is connected-between the-first stage and one ofsaid impedances;

For a better understanding of the invention, together with other and further objects thereof; reference is made to the following description; taken in connection with the accompanying drawing and its scope will be pointed outin the appended claims.

In the accompanying drawing:

Fig. 1 is a schematic diagram of an audioamplifying system embodying the tone control circuit of the present invention;

Fig. 2 is the equivalent circuit of the tone control system of the present invention when adjusted for the reproduction ofthe treble-or higher audio frequencies;

Fig. 3 is the equivalent circuit ofthe tonecontrol system when adjusted for the reproduction of both the high and'the low audio frequencies;

' Fig; 4-is theequivalent circuit of the tone con,- trol'system' of the present invention when adjusted for the reproduction of thebass or lower audio frequencies; and

Fig. 5 is a graph in which curves indicate the over-all audio frequency response of an amplifier embodying the instant invention.

Referring now to Fig. 1 of thedrawing, there is shown an audio amplifier circuit comprising a signal source 8, a first amplifier tube 9, atone control ID, a second amplifiertub-e Handa load circuit l2. The Signal source 8. mayconsist of either the demodulator of a radio broadcast receiver, a phonograph pickup or a microphone,

amplifier, circuit whereby the audio signals applied to tube 9 by the signalsource 8, may be amplified and impressed on tube II for further amplificationf i w The first amplifier tube 9 is provided with a control grid l4, an anode l5, anda cathode l5; which is terminated to ground thro'ugha cathode aha-14a resistor IS. The cathode resistor I8 is also bypassed to ground by a condenser IS. The control grid M of the first amplifier tube 9 is connected, by means of a conductor 20, to the output of the signal source 8. The anode l5 of the first amplifier tube 9 is connected through an anode load resistor 2| to a power input terminal 23 whereby to conventionally provide anode operating potential from a power source, not shown. The anode I5 is also connected by means of a conductor 22 to the tone control I for applying audio signal energy to a control grid 24 of the second amplifier tube II.

The tone control l0 consists of a potentiometer 25 and a pair of condensers 26 and 21. The potentiometer 25 is provided with an upper terminal 28, a lower terminal 29, a tap terminal 30 and a slider arm 3|. The upper terminal 28, the slider arm 3| and the upper terminal of condenser 26 are each connected through the conductor 22 to the anode I for the extraction of audio signal energy therefrom. The lower terminal 29 of potentiometer 25, is coupled to ground through condenser 21 whereas the tap terminal 30 is connected to the lower terminal of condenser 26 and to the grid 24 of tube Understandably, with the slider arm 3| adjusted, as shown, in alignment with the tap terminal 30 the audio signal energy from the anode l5 of tube 9 will be directly impressed upon grid 24 of tube since condenser 26 and portion A of potentiometer 25 are effectively shunted.

The second amplifier tube H is also provided with an anode 33 which is connected through an anode load resistor 34 to the power input terminal 23 for operating potential. Anode 33 is also connected througha coupling condenser 35 to the load circuit I2. Cathode 35 in tube 33 is connected to a cathode resistor 31 which is respectively and are terminated to ground. The I operation of the direct coupled amplifier herein illustrated is conventional and explanation thereof is not required.

Consideration will now be given to the tone control In and more particularly to the potentiometer 25. Experience has shown that a satisfactory tone controlling effect is obtained with a potentiometer having a total resistive value of 1.0 megohm. This resistive value may preferably be divided by the tap terminal 38 into sections A and B having values of 800,00; ohms and 200,000 ohms, respectively. It is to be noted that terminal 30 provides an input circuit the impedance of which remains fixed regardless of the adjustment of slider arm 3|. When the slider arm 3| is arranged in alignment with the tap terminal 30, the slider arm and its connection to conductor 22 short circuits the resistive section A and the condenser 26. It will be noted that movement of the slider arm 3| toward the upper terminal 28 causes an increasingly greater portion of the resistive section A to be inserted between the anode l5 of tube 9 and the control grid 24 of tube At the same time the condenser 25 will shunt that portion of resistive section A which is inserted 4 in this circuit. Inversely, as the slider arm 3| is moved toward the lower terminal 29, it will be apparent that the resistive value of section B will be decreased with respect to the anode circuit of tube 9 and that the reactance of condenser 21 will become effective due to the decreased resistance of section B.

For the purpose of explaining the electrical effects herein achieved, reference is now had to Figs. 2, 3 and 4 of the drawing wherein is shown the equivalent circuits of the tone control apparatus resulting from various settings of the slider arm 3| of the potentiometer 25. There is shown a generator 42 and a resistor R which are representative of the function and the plate resistance respectively of the amplifier tube 9. The resistances, Ra and Rb represent the resistive sections A and B respectively of the potentiometer 25. tion A and the condenser 21 is in series with the resistive section B as shown in Fig. 1. Also shown are output terminals, at point 43 across which appears the audio signal voltage, characteristic of that applied to the grid 24 of the second amplifier tube ll.

Referring to the operation of the tone control I0, assume now that the amplifier circuit is energized and that an audio signal voltage generated by the signal source 8 is applied. to the tube IS.

The tube |5 amplifies this audio signal and impresses it upon the tube through the tone control H). For the purpose of attenuating low frequencies the slider arm 3| is adjusted towards the upper terminal 28 to form the circuit shown in Fig. 2 whereby resistance Ba and condenser 26 are apparent between resistance Rp and the output 43. The signal voltage at the output 43 is determined by plate voltage and is attenuated by resistances Rp, R2. and Rh in which resistance Ra. is shunted, so far as high frequencies are concerned, by the reactance of condenser 26. Eflectively, therefore, when the frequency is such that the reactance of condenser 26 is small compared to the resistance Ra. there is then no high frequency attenuation caused by resistance Ra or condenser 26. Thus, the output voltages depend only on the relationship of resistances R17 and Rh. If, therefore, the value of Rb is large with respect to that of Rp, the effective output voltage is comparable at high frequencies to the input voltage. However at low frequencies the resistance Ra is no longer shunted sufliciently to cancel this attenuation and thus, if frequency decreases the voltage output at points 43 will also decrease accordingly. This attenuation of low frequencies is illustrated in Fig. 5 wherein a curve 44 is indicative of the resultant overall frequency response achieved by the equivalent circuit shown in Fig. 2. Fig. 5 illustrates the voltage output E plotted against frequency f and the curve 44 reveals a decided voltage attenuation at low frequencies and negligible voltage attenuation at higher frequencies.

Both the high and the low audio frequencies may be reproduced, without attenuation, by adjusting the slider arm 3| into alignment with the tap terminal 30. The equivalent circuit of this particular arrangement is illustrated in Fig. 3 and the audio frequency response curve achieved is shown by the curve' 45 of Fig. 5. It will be apparent from the circuit shown in Fig. 3 that the attenuation of the input voltage, as received at the output terminals 43, is determined purely by the ratio of resistance Rp to the resistance Rb which is shunted across the output terminals.

The condenser 26 shunts the resistive sec- The condenser 21 has also been omitted from this equivalent circuit since its capacitance is low compared to the resistance Rb. Therefore since, as was stated for Fig. l, the resistance Rb is relatively large compared to resistance Rp, there is little effective attenuation at any frequency and, as illustrated by the curve 45, the resultant response curve is relatively linear.

For the purpose of attenuating the high frequencies, the slider arm 3! may be moved towards the lower terminal 29 of potentiometer 25 to thereby provide the equivalent circuit illustrated in Fig. 4. The degree of attenuation obtained is shown by the curve 46 of Fig. 5. The resistive section A and the condenser 26 are now relatively ineffective and they can cause no attenuation since there is no load across them. Therefore, those circuit elements have been eliminated from the equivalent circuit shown in Fig. 4. The attenuation of the input voltage as measured at the output terminals 63 is then determined by resistances R Rb and condenser 27. However, since this equivalent circuit portrays the condition existing with the slider arm 3| adjusted to a point below the tap terminal 3!), the resistance Rb is therefore reduced from its previous high value to a point at which the reactance of condenser 21 becomes a decided attenuating factor. Since the reactance of condenser 27 will change with the frequency and become higher for the low frequencies, the effective attenuation induced by resistance Rb and condenser '2? is minimized at the low frequencies and is maximum at the higher frequencies. However, the value of Rb and condenser 21 is now relatively low in comparison with the resistive value R and therefore the attenuation of the high frequencies is considerable.

It will be apparent therefore, from the curves 44, 45 and 56, shown in Fig. 5, and from the foregoing description that the low frequencies may be attenuated without aifecting the high frequencies, that the high frequencies may also be modified without adversely afiecting the low frequencies, and finally, that all frequency discrimination may be removed and linearity of response restored. These separate functions are accomplished with but a single control. While the tone control apparatus herein disclosed may have a wide variety of frequency discriminative characteristics, there follow specifications of a particular tone control arrangement which has proven to be particularly satisfactory.

Section A 800 k.

ohm Potentiometer megohm s t- 1 513 200 k ohms Condenser 26, 0.002 microfarad Condenser 27, 0.02 microfarad While there has been described what is at present considered the referred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a first impedance in series between said stages, a second impedance connected in parallel to said first stage, said first impedance in shunt with at least a portion of said second impedance, conductor means connected between said first impedance, said second stage and a fixed predetermined point on said second impedance, and movable contact means connected between said first stage and said second impedance for varying the proportional value of both of said impedances in said coupling circuit.

2. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a capacitive impedance having a high pass characteristic in a first frequency range and in series between said stages, an impedance connected in parallel to said first stage and having a high pass characteristic in a second range of frequencies including said first frequency range, conductor means adapted to carry both alternating current and direct current connected between said first mentioned impedance, said second stage and a fixed predetermined point on said second mentioned impedance, and adjustable means connected between said first stage and said second mentioned impedance.

3. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a relatively high frequency pass impedance in series between said stages, an impedance connected in parallel to said first stage, conductor means adapted to carry both alternating current and direct current connected between said high pass impedance, said second stage and a fixed predetermined point on said second mentioned impedance, and adjustable means connected between said first stage and said second mentioned impedance for varying the effectiveness of said high pass impedance.

4. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a relatively high frequency pass condenser in series between said stages, an impedance connected in parallel to said first stage, conductor means adapted to carry both alternating current and direct current connected between said high pass condenser, said second stage and a fixed predetermined point on said impedance thereby to provide a fixed impedance across the grid of said second stage, and movable contact means connected between said first stage and said impedance and adjustable for varying the effectiveness of both of said condensers.

5. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a relatively high frequency pass condenser in series between said stages, an impedance connected in parallel to said first stage, conductor means adapted to carry both alternating current and direct cur-- rent connected between said high pass condenser, said second stage and a fixed predetermined point on said impedance thereby to provide a fixed impedance across the grid of said second stage, and movable contact means connected between said first stage and said impedance and adjustable for varying the effectiveness of said high pass condenser.

6. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a

second amplifier stage, a coupling circuit between said stages comprising a relatively high frequency pass condenser in series between said stages, a resistor and condenser connected in series with one another and connected in parallel to said first stage, conductor means adapted to carry both alternating current and direct current connected between said high pass condenser, said second stage and a fixed predetermined point on said resistor thereby to provide a fixed impedance across the grid of said second stage, and movable contact means connected between said first stage and said resistor and adjustable for varying the effectiveness of said high pass condenser.

'7. An audio signal amplifier of the direct coupled type comprising a first amplifier stage, a second amplifier stage, a coupling circuit between said stages comprising a relatively high frequency pass condenser in series between said stages, a resistor and condenser connected in series with one another and connected in parallel to said first stage, conductor means adapted to carry both alternating current and direct current connected between said high pass condenser, said second stage and a fixed predetermined point on said resistor thereby to provide a fixed impedance across the grid of said second stage, and movable contact means connected between said first stage and said resistor and adjustable from one end of said resistor to the other for varying the effectiveness of both of said condensers.

ROBERT J. COWLES.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,938,256 Jacobs Dec. 5, 1933 2,192,959 Ballard Mar. 12, 1940 2,225,337 Fyler Dec. 17, 1940 2,322,558 Bachman June 22, 1943 FOREIGN PATENTS Number Country Date 21,960 Australia Aug. 21, 1929 

